Multiple beams creates a different problem. Say your beam is 1cm across and you are aiming at a 30cm target. All your energy is in that 1cm. You don't much care where in that 30cm it hits so long as it burns through. But with two lasers, to be effective, each beam needs to hit the same sport on the target. Instead of a 30cm target, laser #2 now has to hit the 1cm beam of the first laser.